Molecular dynamics simulation as a complement to diffraction in the study of disorder in crystals

Summary

It is not uncommon that a crystal structure contains a minority component which does not share the overall space-group of the remainder of the structure, but has a lower (or no) effective space-group symmetry. A conventional diffraction treatment will inevitably be made within the space-group of the majority of the system, and will therefore result in an average structure also with this symmetry. We here indicate the value of molecular dynamics (MD) simulation within a supercell of the structure as a complementary tool in providing information concerning the local structure for the minority component. Such information, though illusive, is often the most interesting and the physically most significant aspect of the structure. It can also be crucial information for the testing of new theory, and in the design of new materials. The approach is illustrated for the case of the Na⁺ ion distribution in Na⁺ β′'-alumina, and for the experimentally more inaccessible mixed-ion system Na⁺/Ba²⁺ β′'-alumina.